This is a revised proposal of a competing renewal. Five years of research are proposed in which experiments will be carried out dealing with four specific aims: 1. Biochemical studies of RTs encoded by retrons Ec73 and Ec86; 2. Exploration of possible functions of retrons in E. coli cells; 3. Studies of retrons as mobile elements; 4. In vivo synthesis of oligodeoxyribonucleotides using retrons as a vector.